A message containing letters fromA-Zis being encoded to numbers using the following mapping:

'A' -
>
 1
'B' -
>
 2
...
'Z' -
>
 26

Given an encoded message containing digits, determine the total number of ways to decode it.

For example,
Given encoded message"12", it could be decoded as"AB"(1 2) or"L"(12).

The number of ways decoding"12"is 2.

tag: DP

经典DP，类似stairs

class Solution {
    public int numDecodings(String s) {
        if (s == null || s.length() == 0) return 0;

        int n = s.length();
        int[] dp = new int[n + 1];


        dp[0] = 1;
        if (n >= 1){
            dp[1] = s.charAt(0) == '0' ? 0 : 1;
        }

        for (int i = 2; i <= n; i++){
            int digit1 = Integer.valueOf(s.substring(i - 1, i));
            int digit2 = Integer.valueOf(s.substring(i - 2, i));

            if (digit1 > 0 && digit1 <= 9){
                dp[i] += dp[i - 1];
            }
            if (digit2 >= 10 && digit2 <= 26){
                dp[i] += dp[i - 2];
            }
        }

        return dp[n];
    }
}